This invention relates generally to steam turbine buckets (or blades) and, more particularly, to composite buckets specifically tuned to provide different predetermined frequency damping characteristics and improved system damping.
Steam turbine buckets operate in an environment where they are subject to high centrifugal loads and vibratory stresses. Vibratory stresses increase when bucket natural frequencies become in resonance. The magnitude of vibratory stresses when a bucket vibrates in resonance is proportional to the amount of damping present in the system (damping is comprised of material, aerodynamic and mechanical components), as well as the vibration stimulus level. For continuously coupled buckets, the frequency of vibration is a function of the entire system of blades in a row, and not necessarily that of individual blades within the row.
At the same time, centrifugal loads are a function of the operating speed, the mass of the bucket, and the radius from engine centerline where that mass is located. As the load (mass) of the bucket increases, the physical area or cross-sectional area must increase at lower radial heights to be able to carry the mass above it without exceeding the allowable stresses for the given material. This increasing section area of the bucket at lower spans contributes to excessive flow blockage at the root and thus lower performance. The weight of the bucket contributes to higher disk stresses and thus to potentially reduced reliability.
Several prior U.S. patents relate to so-called “hybrid” bucket designs where portions of the airfoil portion are composed of a combination of a metal and a polymer filler material. These prior patents include U.S. Pat. Nos. 6,139,278; 6,042,338; 5,931,641 and 5,720,597.